Shunt resistor

ABSTRACT

A shunt resistor the resistive value of which can be lowered simply and easily has: a first resistive body, two base materials that sandwich the first resistive body therebetween and are joined by a welding to the first resistive body, and a second resistive body joined by a welding to the two base materials at different positions from the first resistive body. In addition, the second resistive body can come into contact with the first resistive body.

FIELD OF THE INVENTION

The present invention relates to a shunt resistor.

BACKGROUND OF THE INVENTION

A shunt resistor as described in Japanese unexamined patent applicationpublication No. 2009-244065 is known as a conventional shunt resistor.As shown in FIG. 7 , the conventional shunt resistor 100 includes twoplate-like base materials 102 sandwiching a resistive body 101therebetween and joined to the resistive body 101 by welding 101 a theresistive body 101 and a measurement terminal portion 103 joined to eachof the base materials 102 by welding 103 a. Reference sign 102 a denotesa circular bolt hole for passing a shaft portion of a bolt (not shown).

SUMMARY OF THE INVENTION

Incidentally, when a current flows through the shunt resistor 100 asdescribed above, the resistive body 101 generates heat, and theresistance value of the resistive body 101 changes depending on the heatgeneration temperature. Therefore, the resistance value of the resistivebody 101 needs to be lowered so as not to raise the heat generationtemperature. For example, the resistance value of the resistive body 101is preferably about 100μΩ when a large current of 400 A flows throughthe shunt resistor 100. The resistance value of the resistive body 101is preferably about 30μΩ to 50μΩ when a large current of 1000 A flowsthrough the shunt resistor 100.

Accordingly, in order to lower the resistance value of the resistivebody 101, it is conceivable to narrow the width W10 (see FIG. 7(b)) ofthe resistive body 101. For example, the width W10 (see FIG. 7(b)) ofthe resistive body 101 needs to be about 8 mm in order to set theresistance value of the resistive body 101 to 100μΩ, and the width W10(see FIG. 7(b)) of the resistive body 101 needs to be about 3 mm inorder to set the resistance value of the resistive body 101 to 30μΩ to50μΩ.

However, there has been a problem that when the width W10 (see FIG.7(b)) of the resistive body 101 is narrowed as described above, it isvery difficult to join the resistive body 101 and the two base materials102 by welding 101 a the resistive body 101 with the resistive body 101sandwiched between the two base materials 102. Therefore, there has beena problem that it is very difficult to lower the resistance value.

On the other hand, it is known that the resistance value of theresistive body 101 changes depending on the volume of the resistive body101 itself. Accordingly, a method is generally known that a notch 101 bformed in a concaved shape is provided to the resistive body 101 asshown in FIG. 7 to reduce the volume of the resistive body 101 itself,thereby adjusting the resistance value of the resistive body 101.

However, while the method as described above can increase the resistancevalue the resistance value cannot be lowered. Therefore, there has beena problem that the resistance value cannot be lowered by this method aswell.

Accordingly, in view of the foregoing problems, an object of the presentinvention is to provide a shunt resistor whose resistance value can belowered simply and easily.

The foregoing object of the present invention is achieved by thefollowing means. It is noted that reference signs of embodiments to bedescribed later are added in parentheses, but the present invention isnot limited thereto.

According to the first aspect of the present invention, a shunt resistoris characterized by including a first resistive body (10),

two base materials (11) sandwiching the first resistive body (10)therebetween and joined to the first resistive body (10) by welding(Y1), and

a second resistive body (12, 12B, 12C, 12D) joined to the two basematerials (11) by welding (Y2, Y2B, Y2C, Y2D) at a position differentfrom the first resistive body (10).

Further, according to the second aspect of the present invention, theshunt resistor according to the above first aspect is characterized inthat the second resistive body (12, 12B, 12C) comes in contact with thefirst resistive body (10) and is joined to the two base materials (11)by welding (Y2, Y2B, Y2C).

On the other hand, according to the third aspect of the presentinvention, the shunt resistor according to the above first aspect ischaracterized in that the second resistive body (12D) is joined to thetwo base materials (11) by welding (Y2D) at a position adjacent to thefirst resistive body (10) and in a non-contact manner with the firstresistive body (10).

Next, advantageous effects of the present invention will be describedwith reference signs in the drawings. It is noted that reference signsin the embodiments to be described later are added in parentheses, butthe present invention is not limited thereto.

According to the first aspect of the present invention, a joining areaof the resistive bodies (the first resistive body 10 and the secondresistive body 12, 12B, 12C, 12D) joined to the two base materials (11)is increased by joining the second resistive body (12, 12B, 12C, 12D) bywelding (Y2, Y2B, Y2C, Y2D) to the two base materials (11) sandwichingthe first resistive body (10) therebetween and joined to the firstresistive body (10) by welding (Y1), at a position different from thefirst resistive body (10). This makes it possible to lower theresistance value.

Thus, according to the present invention, the resistance value can belowered only by joining the second resistive body (12, 12B, 12C, 12D) tothe two base materials (11) by welding (Y2, Y2B, Y2C, Y2D) at a positiondifferent from the first resistive body (10), so that the resistancevalue can be lowered simply and easily.

Further, when the second resistive body (12, 12B, 12C, 12D) is joined tothe two base materials (11) by welding (Y2, Y2B, Y2C, Y2D), it ispreferable to bring the second resistive body (12, 12B, 12C) intocontact with the first resistive body (10) and join the second resistivebody (12, 12B, 12C) to the two base materials (11) by welding (Y2, Y2B,Y2C) as in the second aspect of the present invention. Alternatively, asin the third aspect of the present invention, it is preferable to jointhe second resistive body (12D) to the two base materials (11) bywelding (Y2D) at a position adjacent to the first resistive body (10)and in a non-contact manner with the first resistive body (10).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(a) is a perspective view of a shunt resistor according to anembodiment of the present invention and FIG. 1(b) is a plan view of theshunt resistor according to the same embodiment.

FIG. 2 is a plan view of a shunt resistor according to anotherembodiment.

FIG. 3 is a plan view of a shunt resistor according to still anotherembodiment.

FIG. 4 is a bottom view of a shunt resistor according to further anotherembodiment.

FIG. 5 is a plan view of a shunt resistor according to still furtheranother embodiment.

FIG. 6(a) is a plan view of a base material according to an example,FIG. 6(b) is a plan view of a first resistive body and a secondresistive body according to the example, and FIG. 6(c) is a plan viewshowing experimental conditions of a shunt resistor according to theexample.

FIG. 7(a) is a perspective view of a conventional (prior art) shuntresistor and FIG. 7(b) is a plan view of the conventional shuntresistor.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, an embodiment of a shunt resistor according to the presentinvention will be specifically described with reference to the drawings.It is noted that, in the following description, when vertical andhorizontal directions are indicated, it shall mean vertical andhorizontal directions when viewed from the front of the figure.

A shunt resistor according to the present embodiment is used inmeasuring the current value of a current path through which a largecurrent flows from a battery for high-voltage applications or aninverter to a motor circuit that are used in, for example, electricvehicles (EVs), hybrid vehicles (HVs), and plug-in hybrid vehicles(PHVs). As shown in FIG. 1 , the shunt resistor 1 is composed of a firstresistive body 10, two base materials 11 sandwiching the first resistivebody 10 therebetween and integrally formed with the first resistive body10 by welding Y1, a second resistive body 12 joined to the two basematerials 11 by welding Y2 at a position different from the firstresistive body 10, and a measurement terminal 13 erected and fixed bywelding Y3 on each of upper surfaces 11 a of the two base materials 11.

The first resistive body 10 is the same as a resistive body 101 shown inFIG. 7 , and as shown in FIG. 1(b), is formed, for example, in a thickplate shape having a thickness of 3 mm to 5 mm and in a shortrectangular shape consisting of an upper surface 10 a, a lower surface10 b, an upper side surface 10 c, a lower side surface 10 d, a left sidesurface 10 e, and a right side surface 10 f, and is formed with the samewidth W10 as the width W10 shown in FIG. 7(b). Further, the firstresistive body 10 is formed of, for example, a Cu—Mn-based alloy, aCu—Ni-based alloy, or a Ni—Cr-based alloy.

As shown in FIG. 1 , in the first resistive body 10 thus formed, thebase material 11 located on the left side in the figure is joined to theleft side surface 10 e of the first resistive body 10 by welding Y1 andthe base material 11 located on the right side in the figure is joinedto the right side surface 10 f of the first resistive body 10 by weldingY1. As a result, the pair of base materials 11 are integrally formedwith the first resistive body 10 in a manner of sandwiching the firstresistive body 10 therebetween.

The base material 11 is the same as a base material 102 shown in FIG. 7and is a so-called bus bar. The base material 11 is formed of metal suchas copper, and as shown in FIG. 1 , is formed, for example, in a thickplate shape having a thickness of about 3 mm to 5 mm and in a longrectangular shape consisting of the upper surface 11 a, a lower surface11 b, an upper side surface 11 c, a lower side surface 11 d, one sidesurface 11 e, and the other side surface 11 f. A circular bolt hole 11 gfor passing a shaft portion of a bolt (not shown) is formed verticallypenetratingly on the other side surface 11 f side (the side opposite tothe joint part of the first resistive body 10) along the width directionof the base material 11.

Thus, as shown in FIG. 1 , in the base materials 11 thus formed, oneside surface 11 e of the base material 11 located on the left side ofthe figure is joined to the left side surface 10 e of the firstresistive body 10 by welding Y1, and one side surface 11 e of the basematerial 11 located on the right side of the figure is joined to theright side surface 10 f of the first resistive body 10 by welding Y1. Asa result, the pair of base materials 11 and the first resistive body 10are integrally formed by welding Y1.

As shown in FIG. 1 , the second resistive body 12 is formed, forexample, in a thick plate shape having a thickness of about 3 mm to 5 mmand in a short rectangular shape, and is formed of, for example, aCu—Mn-based alloy, a Cu—Ni-based alloy, or a Ni—Cr-based alloy.

Thus, as shown in FIG. 1(b), in the second resistive body 12 thusformed, a lower side surface 12 b of the second resistive body 12 isjoined to the upper side surfaces 11 c of the pair of base materials 11by welding Y2 so that an upper surface 12 a of the second resistive body12 and the upper surface 10 a of the first resistive body 10 are flushwith each other and the lower side surface 12 b of the second resistivebody 12 and the upper side surface 10 c of the first resistive body 10come in contact with each other. As a result, the pair of base materials11 and the second resistive body 12 are integrally formed by welding Y2.

The measurement terminal 13 is the same as a measurement terminalportion 103 shown in FIG. 7 and is capable of mounting thereon a printedcircuit board for current detection. The measurement terminal 13 isformed of, for example, copper or tin plating, and as shown in FIG. 1 ,is erected and fixed on each of the two base materials 11 by welding Y3.

Thus, according to the present embodiment described above, the joiningarea of the resistive bodies (the first resistive body 10 and the secondresistive body 12) joined to the two base materials 11 is increased byjoining the second resistive body 12 by welding Y2 to the two basematerials 11 sandwiching the first resistive body 10 therebetween andintegrally formed with the first resistive body 10 by welding Y1, at aposition different from the first resistive body 10. This makes itpossible to lower the resistance value.

That is, when the current value of a current path through which a largecurrent flows from a battery for high-voltage applications or aninverter to a motor circuit is measured, the current flows through thetwo base materials 11. When the joining area of the resistive bodies(the first resistive body 10 and the second resistive body 12)increases, the current path increases accordingly. Thus, when thecurrent path increases, the current flowing through the resistive bodies(the first resistive body 10 and the second resistive body 12) flowseasier as compared with the conventional case, and the heat generationtemperature of the resistive bodies (the first resistive body 10 and thesecond resistive body 12) can be made lower than before. This makes itpossible to lower the resistance value.

Thus, according to the present embodiment, the resistance value can belowered only by joining the second resistive body 12 to the two basematerials 11 by welding Y2 at a position different from the firstresistive body 10 without narrowing the width W10 (see FIG. 1(b)) of thefirst resistive body 10, that is, in the same width W10 (see FIG. 1(b)and FIG. 7(b)) as the conventional one. Therefore, the resistance valuecan be lowered simply and easily.

Further, according to the present embodiment, when the resistance valueis to be increased in adjusting the resistance value, the resistancevalue is simply required to be increased by using the method as shown inFIG. 7 and providing the first resistive body 10 and/or the secondresistive body 12 with a concave-shaped notch to reduce the volume ofthe first resistive body 10 and/or the second resistive body 12themselves/itself. On the other hand, when the resistance value is to belowered, a new second resistive body having a large area, such as makingit longer than the second resistive body 12, is simply required to bejoined to the two base materials 11 by welding Y2.

Thus, according to the present embodiment, the resistance value can beadjusted simply and easily.

Further, when the resistance value is lowered, the heat generationtemperature of the resistive bodies (the first resistive body 10 and thesecond resistive body 12) can be made lower than before. Thus, the heatgeneration loss can be reduced.

Further, when the printed circuit board for current detection is mountedon the measurement terminal 13, a heat radiation fin is required.However, when the resistance value is lowered, the heat generationtemperature of the resistive bodies (the first resistive body 10 and thesecond resistive body 12) can be made lower than before, so that theheat radiation fin can be made compact or unnecessary.

Incidentally, the shape of the shunt resistor 1 shown in the presentembodiment is merely an example, and various modifications and changescan be made within the scope of the gist of the present inventiondescribed in the claims. For example, the example in which the firstresistive body 10 and the second resistive body 12 are providedseparately is shown in the present embodiment, but without being limitedthereto, they may be integrated. However, it is preferable to providethem separately because only the second resistive body 12 is simplyrequired to be changed when the resistance value is to be lowered inadjusting the resistance value.

On the other hand, the shape of the second resistive body 12 may be anyshape, and the position of the joint to the two base materials 11 is notlimited to that shown in FIG. 1 and may be any position. For example, ashunt resistor 1A as shown in FIG. 2 may be used. This point will bespecifically described with reference to FIG. 2 . The sameconfigurations as those of the shunt resistor 1 shown in FIG. 1 aredenoted by the same reference signs and their detailed description willbe omitted.

The difference between the shunt resistor 1A shown in FIG. 2 and theshunt resistor 1 shown in FIG. 1 is only that another second resistivebody 12 is newly joined, while all else is the same. That is, as shownin FIG. 2 , another second resistive body 12 is joined to the pair ofbase materials 11 by joining an upper side surface 12 c of the secondresistive body 12 to the lower side surfaces 11 d of the pair of basematerials 11 by welding Y2 so that an upper surface 12 a of the secondresistive body 12 and the upper surface 10 a of the first resistive body10 are flush with each other and the upper side surface 12 c of thesecond resistive body 12 and the lower side surface 10 d of the firstresistive body 10 come in contact with each other. As a result, the pairof base materials 11 and the pair of second resistive bodies 12 areintegrally formed by welding Y2.

Thus, in this manner as well, the current path increases, and thecurrent flowing through the resistive bodies (the first resistive body10 and the second resistive body 12) flows easier as compared with theconventional case. Accordingly, the heat generation temperature of theresistive bodies (the first resistive body 10 and the second resistivebody 12) can be made lower than before, and the resistance value can belowered. When the resistance value is to be lowered in adjusting theresistance value, another resistive body 12 may be newly joined to thetwo base materials 11 by welding Y2 as shown in FIG. 2 , instead ofreplacing the second resistive body 12 itself.

Further, a shunt resistor 1B shown in FIG. 3 may be used. This pointwill be specifically described with reference to FIG. 3 . The sameconfigurations as those of the shunt resistor 1 shown in FIG. 1 aredenoted by the same reference signs and their detailed description willbe omitted.

The difference between the shunt resistor 1B shown in FIG. 3 and theshunt resistor 1 shown in FIG. 1 is only that the shape of the secondresistive body and the joint position are different, while all else isthe same. That is, as shown in FIG. 3 , a second resistive body 12B isformed in a substantially rectangular shape in plan view, a first notch12Ba1 having a leftward U shape in plan view is provided on a left sidesurface 12Ba side, and a second notch 12Bb1 having a rightward U shapein plan view is provided on a right side surface 12Bb side. The secondresistive body 12B thus formed is placed on the upper surface 10 a ofthe first resistive body 10 so as to be orthogonal to the firstresistive body 10, and parts thereof in contact with the pair of basematerials 11 (the left side surface 12Ba side and the right side surface12Bb side of the second resistive body 12B) are joined by welding Y2B.As a result, the pair of base materials 11 and the second resistive body12B are integrally formed by welding Y2B. At this time, the pair ofmeasurement terminals 13 are positioned in the first notch 12Ba1 and thesecond notch 12Bb1 respectively so as to prevent the possibility thatthe second resistive body 12B and the measurement terminals 13 come incontact and the measurement terminals 13 measure an erroneous currentvalue.

Thus, in this manner as well, the current path increases and the currentflowing through the resistive bodies (the first resistive body 10 andthe second resistive body 12B) flows easier as compared with theconventional case. Therefore, the heat generation temperature of theresistive bodies (the first resistive body 10 and the second resistivebody 12B) can be made lower than before and the resistance value can belowered.

On the other hand, a shunt resistor 1C as shown in FIG. 4 can be used.This point will be specifically described with reference to FIG. 4 . Thesame configurations as those of the shunt resistor 1 shown in FIG. 1 aredenoted by the same reference signs and their detailed description willbe omitted.

The difference between the shunt resistor 1C shown in FIG. 4 and theshunt resistor 1 shown in FIG. 1 is only that the shape of the secondresistive body and the joint position are different, while all else isthe same. That is, as shown in FIG. 4 , a second resistive body 12C isformed in a substantially rectangular shape in plan view. The secondresistive body 12C thus formed is placed on the lower surface 10 b ofthe first resistive body 10 so as to be orthogonal to the firstresistive body 10, and parts thereof in contact with the pair of basematerials 11 (a left side surface 12Ca side and a right side surface12Cb side of the second resistive body 12C) are joined by welding Y2C.As a result, the pair of base materials 11 and the second resistive body12C are integrally formed by welding Y2C.

Thus, in this manner as well, the current path increases and the currentflowing through the resistive bodies (the first resistive body 10 andthe second resistive body 12C) flows easier as compared with theconventional case. Therefore, the heat generation temperature of theresistive bodies (the first resistive body 10 and the second resistivebody 12C) can be made lower than before and the resistance value can belowered.

Incidentally, in any of the shunt resistor 1 shown in FIG. 1 , the shuntresistor 1A shown in FIG. 2 , the shunt resistor 1B shown in FIG. 3 ,and the shunt resistor 1C shown in FIG. 4 , the example in which thefirst resistive body 10 and the second resistive body 12, 12B, 12C arein contact with each other is shown. However, they do not have to be incontact with each other as shown in a shunt resistor 1D shown in FIG. 5. This point will be specifically described with reference to FIG. 5 .The same configurations as those of the shunt resistor 1 shown in FIG. 1are denoted by the same reference signs and their detailed descriptionwill be omitted.

The difference between the shunt resistor 1D shown in FIG. 5 and theshunt resistor 1 shown in FIG. 1 is only that the shape of the secondresistive body and the joint position are different, while all else isthe same. That is, as shown in FIG. 5 , a second resistive body 12D isformed in a downward U shape in plan view, has an upper surface 12Dathereof flush with the upper surfaces 11 a of the pair of base materials11 and has a lower side surface 12Db thereof joined to the upper sidesurfaces 11 c of the pair of base materials 11 by welding Y2D. As aresult, the pair of base materials 11 and the second resistive body 12Dare integrally formed by welding Y2D. At this time, the first resistivebody 10 and the second resistive body 12D are located on adjacentpositions but are in a non-contact state.

Thus, in this manner as well, the current path increases and the currentflowing through the resistive bodies (the first resistive body 10 andthe second resistive body 12D) flows easier as compared with theconventional case. Therefore, the heat generation temperature of theresistive bodies (the first resistive body 10 and the second resistivebody 12D) can be made lower than before and the resistance value can belowered.

The shunt resistors 1, 1A, 1B, 1C, 1D exemplified above can be appliedalso as a shunt resistor for a fuse requiring low resistance.

Next, the present invention will be described in more detail using anexample.

10 shunt resistors 1 shown in FIG. 1 and 10 conventional shunt resistorswere used to measure the resistance value.

As the size of the shunt resistor 1, as shown in FIG. 6(a), a basematerial 11 was used in which the width of the base material 11(distance from the other side surface 11 f to one side surface 11 e ofthe base material 11) was 38 mm, the height of the base material 11(distance from the upper side surface 11 c to the lower side surface 11d of the base material 11) was 18 mm, the distance from the upper sidesurface 11 c of the base material 11 to the center point of the bolthole 11 g of the base material 11 was 9 mm, the distance from the otherside surface 11 f of the base material 11 to the center point of thebolt hole 11 g of the base material 11 was 12 mm, the diameter of thebolt hole 11 g of the base material 11 was 8.0 mm, and the thickness was3.0 mm.

Further, a first resistive body 10 having a width of 8.0 mm, a height of18 mm, and a thickness of 2.0 mm was used as shown in FIG. 6(b). Asecond resistive body 12 having a width of 16 mm, a height of 2.0 mm,and a thickness of 2.0 mm was used as shown in FIG. 6(b).

It is noted that the conventional shunt resistor is only not providedwith the second resistive body 12, while all else is the same as thebase material 11 and the first resistive body 10.

With respect to the shunt resistor 1 formed in such a size, as shown inFIG. 6(c), a current was applied to the other side surface 11 f side(see black circle in the figure) of the base material 11 so that thecurrent flew from the right side to the left side in the figure, and themeasurement terminals 13 were arranged so that the voltage detectionpitch was 12 mm. The conventional shunt resistors were also tested underthe same conditions.

Under such conditions, the shunt resistors 1 and the conventionalresistors were each placed on a resistance measuring jig manufactured bySUNCALL CORPORATION, and the resistance value was measured using aresistance measuring instrument, RM3543 RESISTANCE HiTESTER,manufactured by HIOKI E.E. CORPORATION. The results are shown in Table1.

TABLE 1 Resistance value (mΩ) Comparative Example No. Example(Conventional)  1 0.08432 0.09912  2 0.08423 0.09935  3 0.08501 0.09972 4 0.08458 0.09943  5 0.08425 0.09978  6 0.08448 0.09929  7 0.084050.09937  8 0.08443 0.09962  9 0.08477 0.09955 10 0.08457 0.09971

As shown in the results of Table 1 above, it can be seen that theresistance values of the shunt resistors 1 are all lower than those ofthe conventional shunt resistors. Further, the average value of theresistance values of the conventional shunt resistors is 0.09949 mΩ,whereas the average value of the resistance values of the shuntresistors 1 is 0.08446 mΩ. From this, it can be seen that the shuntresistor 1 has the resistance value about 15% lower than that of theconventional shunt resistor.

Then, it was found that the resistance value can be lowered when theshunt resistor according to the present embodiment is used. Thus,according to the present embodiment, the resistance value can be loweredsimply and easily.

What is claimed is:
 1. A shunt resistor comprising: a first resistivebody, and the first resistive body being rectangular; two base materialssandwiching the first resistive body therebetween and joined to thefirst resistive body by welding, and the two base materials beingrectangular; a second resistive body joined to the two base materials bywelding at a position different from the first resistive body, and thesecond resistive body being rectangular; and a lower side surface of thesecond resistive body is joined to an upper side surfaces of the twobase materials by welding so that an upper surface of the firstresistive body and an upper surface of the second resistive body areflush with each other and the lower side surface of the second resistivebody and an upper side surface of the first resistive body come incontact with each other, or an upper side surface of the secondresistive body is joined to lower side surfaces of the two basematerials by welding so that the upper surface of the first resistivebody and the upper surface of the second resistive body are flush witheach other and the upper side surface of the second resistive body and alower side surface of the first resistive body come in contact with eachother.
 2. A shunt resistor comprising: a first resistive body, and thefirst resistive body being rectangular; two base materials sandwichingthe first resistive body therebetween and joined to the first resistivebody by welding, and the two base materials being rectangular; a secondresistive body joined to the two base materials by welding at a positiondifferent from the first resistive body, and the second resistive bodybeing rectangular; a third resistive body joined to the two basematerials by welding at a position different from the first rectangularresistive body, and the third resistive body being rectangular; a lowerside surface of the second resistive body is joined to an upper sidesurfaces of the two base materials by welding so that an upper surfaceof the first resistive body and an upper surface of the second resistivebody are flush with each other and the lower side surface of the secondresistive body and an upper side surface of the first resistive bodycome in contact with each other, and an upper side surface of the thirdresistive body is joined to lower side surfaces of the two basematerials by welding so that the upper surface of the first resistivebody and the upper surface of the third resistive body are flush witheach other and the upper side surface of the third resistive body and alower side surface of the first resistive body come in contact with eachother.